Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...

Cheats

Item Duplication (Cooperative Mode)

You can perform this cheat anywhere in the game, but it works best if you put it to use near a checkpoint and bench. You'll need to be playing with a friend, preferably one who is equipped with a headset so that you can communicate as needed. Start by giving your friend the items you wish to duplicate. After the trade is completed, exit out to the main menu and then immediately have the friend who received your loot save and quit from the pause menu. The trick only works if you follow that precise order, and quickly. If you did it properly, both players should now have the desired items in their inventory.

Item Exploit

When picking up an artifact, advance to the next location to go past a doorway. Once you have passed the door, let it shut and reopen. Now return to the location where you found the artifact and in its place you should find resources such as the semiconductors, tungsten, scrap metal, transoucer and health packs. You can repeat this trick as often as you like.

Unlockables

Unlock Retro Mode

To unlock Retro mode, you must complete the game while on "Hardcore Mode"

Log Collection Unlocks

You'll gain access to additional gear by collecting the various logs in Dead Space 3.
MK-II Overclocked Module Set - Collecting all Logs
Research Circuit Set - Collect all Text Logs
Comms Circuit Set - Collect all Audio Logs

Devil's Horns

Completing the New Game+ mode on the "Classic" setting will unlock devil's horns.

Post-Game Content

Complete the game on various difficulty settings to gain access to additional modes and content.
New Game+ - Completing the game once on any difficulty
Retro Mode - Completing the game in Hardcore Mode
Dead Space Legends Suit Set - Complete the game once
+3 CLP Circuit Set - Complete the game on Casual difficulty
+3 RLD Circuit Set - Complete the game on Normal difficulty
+3 SPD Circuit Set - Complete the game on Hard difficulty
+3 DMG Circuit Set - Complete the game on Impossible difficulty